1. Field of the Invention
This invention relates to a casting technique using resin cores and, more particularly, to a casting technique with an improved step of removing a resin core from a cast product.
2. Description of the Prior Art
In casting, cores are used to cast hollow products. The core should have a mechanical strength sufficient to maintain its shape against the heat and pressure of molten metal during the casting. In addition, it is required to have a readily breaking property, that is, it should be comparatively readily broken to permit its ready removal from the cast product after the casting. Currently, sand cores which are formed by using sand and a thermosetting resin are extensively used.
The sand core has such disadvantages that its preparation requires many steps and that it is readily damaged when the casting pressure is increased during the casting, and there are proposals of using resin cores in lieu of sand cores.
The resin core is formed by using a thermoplastic resin, and it can satisfy the following two properties when the proper type of resin to be used is selected.
The first one of the properties is that the resin core maintains sufficient mechanical strength to maintain its shape against the heat and pressure of the molten metal poured into the casting die until the molten metal is solidified.
The second property is that the resin core that is accommodated in the cast product is melted when its temperature is further increased after the solidification of the molten metal.
When these two properties are satisfied, a hollow space having an accurate shape is formed inside the cast product, and moreover, the molten resin core is readily removed from the cast product.
In the prior art, however, in the case of removing the resin core from the cast product by melting the core, when the shape of the hollow space is complicated, the molten resin may partly remain in the cast product.